Cracking hydrocarbon gases in the presence of finely divided coke



June 22, 1948. M H, ARvEsoN 2,443,714

cnAcxING mmnocmou GASES IN THE PRESENCE oF FINELY DIVIDED com Filed Deo. 31, 1

QW. RWNW m NNN ,Patented June 22,1948

CRACKING HYDROCARBON GASES IN THE PRESENCE F FINELY DIVIDED COKE Maurice H. Arveson, Flossmoor, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application December 31, 1940, Serial No. 372,517

' carbon conversion a portion of the 4feed stock is reduced to carbon, and the carbon deposited on the catalyst surface, thus diminishing or obscurlng its eiectiveness. If the catalyst is to be reused the carbon deposit must be removed. Catalysts for cracking usually include solid refractory materials, such as clays, metal oxides, etc. Solid carbonaceous materials on the other hand, while not exhibiting the same catalytic eiect as the usual refractory materials, are not deteriorated to the same extent by carbon deposition.

It is an object of this invention to provide an improved method and means for the cracking of hydrocarbon gases in the presence of a solid carbonaceo'us material. Another object of my invention is to provide an improved process for the conversion o gaseous hydrocarbons by cracking in the presence of coke and restoration of the coke for reuse.

A further object of my invention is to provide method and means for the cracking of normally gaseous hydrocarbons in which coke from cracking is recycled to a cracking step with intermediate heating. A particular object of my invention :is to provide a method and means for converting normally gaseous hydrocarbons to unsaturated hydrocarbons and hydrocarbons of low molecular Weight in the presence of coke and the restoration of the coke for reuse in the cracking operation.

It is an important object of this invention to provide an improved method whereby hydrocarbon gases can be converted to other products at high temperature in which indirect` heating through metal walls at temperatures higher than a few hundred degrees is eliminated and yet the process can be operated at temperatures of 2000" F. and even higher.

It is the further object of this invention to provide a process for the conversion of normally gaseous hydrocarbons wherein heat generated in the system is used to supply the heat necessary to 2 Claims. (Cl. 26d-683) raise the hydrocarbons to reaction temperature and supply the heat of cracking without the necessity of transferring this heat through metal walls.

Other objects and advantages of my invention will become apparent as the description thereof proceeds read in conjunction with the accompanying drawing which is a simple flow diagram illustrating apparatus suitable for carrying out one embodiment of my invention.

Brieily stated, my invention contemplates contacting normally gaseous hydrocarbons with powdered coke, coal, lignite, or other solid carbonaceous material, at an elevated temperature and low or moderate pressures in a low velocity upow reactor, separation of the reaction products from the carbonaceous material and the reheating of the separated coke in a low velocity upow coke heater with recirculation of the coke to the cracking operation.

Referring now to the drawing: Feed stock from any suitable source (not shown) enters through line I0 and pump Il and passes to coils l2 in furnace I3. As a feed stock I can employ a mixture oi' hydrocarbon gasesisuch as butane, propane and ethane, or I can use an individual hydrocarbon in a more or less puried state. The hot gases from coil l2 pass through conduit I4 and are contacted with hot coke particles in powdered form from line l5, the rate of addition being regulated by valve 15a which can, if desired, be responsive to the iiow of uids through conduit i4 or to the temperature in the reaction zone. The mixed hot gases and hot coke particles are passed via line I6 to low velocity, upflow cracking chamber I1. Cracking chamber Il is designed to be only so large as is necessary in order to obtain the required amount of contact for heat transfer and contact time in order to eiect the conversion desired. In general, the time during which the feed stock is maintained in the reaction chamber should be quite short.

Reaction chamber l1 is operated in general at temperatures of about 1100 F. to about 1500 F. or higher, depending upon the type of feed stock undergoing conversion, the lower the molecular weight of the predominating hydrocarbons, the higher temperatures required. Although the feed stock and coke can be heated to approximately the same required temperature before mixing, I preier that the temperature ofthe coke be` distinctly above that of the hydrocarbons, and the two mixed in-such proportions that the desired temperature is maintained in reaction chamber I1. The hydrocarbon feed can be heated to an 3 elevated temperature but yet a temperature below that at which a substantial amount of cracking or decomposition takes place, as, for instance, about 800 to about 1000 F. In this way, coking or carbon deposition in furnace coils i2 is minimized or avoided. For example, a feed containing predominantly propane can be heated to 1000 F. in furnace I3, and then blended with coke from line I5 which is at a temperature of 1800 F'. until the blend in reaction chamber I1 is about 1500 F.

The ratio of coke to feed can vary over wide limits, dictated chiefly by the temperatures to which the feed stockl and coke are heated and the temperature at which the reaction is to be carried out, but generally a weight ratio of coketo-gas of from about 1:1 to about 15:1 is preferred.

When finely divided carbonaceous material such as coke is introduced at a fairly constant rate in the base of a vertical reactor. wherein there is an upwardly flowing gas stream and the superficial velocity of said stream is varied it will be found that at high velocities the coke moves through the reactor-at substantially the same velocity as the gas stream, i. e., there is not a great tendency towards settling. At very low superficial gas velocities through the reactor the coke may settle out of the gas and assume a quiescent state. At intermediate gas velocities the coke will be vcarried upwardly with the gas stream but there will be a pronounced tendency toward settling or slipping, i. e. the coke will move upwardly in the reactorat a much lower velocity than the supporting stream. I refer to the use of this intermediate velocity action as hindered settling.

Due to the velocity of the gas -stream and the presence of the finely divided solid, the interior of the reactor .presents an appearance of turbulence or boilin-g. The particles tend to settle to the base of the reactor but due to the upwardly rising gas stream they are continually being lifted within the reactor, until nally they are discharged with the gas stream. 'I'his 'turbulent effect tends to distribute the heat evenly through; out the reactor from top to bottom, and also equally among all of the ilne particles.

In operating reaction chamber I1 I prefer to employ a gas contact time of from about 0.5 to about 50 seconds, preferably about 5 seconds, and a coke residence time of from about 0.5 to 100 minutes, preferably about 21/2 minutes, although I can also carry my process without any hindered settling, the gas and coke passing through the reaction chamber simultaneously and without any separation. In general, the ratio of the catalyst residence time to gas contact time will be in the range from 1.5:1 to 40:1. 'The pressure will be near `atmospheric or suflicientlyv higher to furnish the pressurev necessary to force the reaction products through the system.

As an alternate method of operation, hot coke particles can be added directly to reaction cham- Y ber I1 rather than to the feed gases in line I4 and at not too great a height in reaction chamber I1 so that all of the contact between the hot coke particles and the feed stock takes place within reaction chamber l1. Accordingly, hot coke is added from line I8 controlled by valve I9. In this manner hotter coke can be added than might be added to the feed in line I4, due to the limitations imposed by the materials available for transfer lines etc. In other words, the addition of coke at, say, 2500 F. at a point in a steel transfer line might cause a hot spot at that point which the feed gases could not dissipate suiii Aciently rapidly to avoid overheating of the metal.

On the other hand. a short line fromv the hot coke source to the reaction chamber, as will other parts of the equipment, can suitably be lined with a heat-resistant material such as lire brick or ceramic insulation, and no harm results from the elevated temperatures. In addition, the introduction of coke at very elevated temperatures reduces the amount of recirculation necessary, the nature of the agitated gases in the reaction chamber plus the diluting effect of the solid coke already .present tending to maintain a uniform temperature throughout the vessel, thus permitting smaller amounts of hotter coke to raise the temperature of the feed and supply the heat of cracking without local hot spots where Aovercraclting and excess degradation to carbon difference in the coke-particle size throughout the mass, the coke can be withdrawn from cyclone separator 2| by opening valve 22 in line 23 which leads to hopper 24 and standpipe 25, while the gases carrying additional coke particles pass overhead through line 26.to cyclone separator 21 wherein further quantities of solid material are recovered, passing via line 28 to hopper 24. If necessary a third cyclone separator or precipitator 2' can be employed, the hot gases from cyclone separator 28 together with any remaining coke passing overhead through line 30 to cyclone separator or precipitator 29, while the nal colte particles are recovered and directed to hopper 24 by line 3l.

During the conversion of the feed stock in the process, various amounts of carbon are deposited on the solid particles so that the coke particles grow in size during the cracking of the stock. I'he coke particles will be reduced in size to some extent during the heating step. (to be described) but they are not uniformly and continuously returned to their original size and shape. Accordingly, it becomes desirable to restore the coke particles to their approximate original .particle size, preferably by mechanical means. 'Since the coarsest particles will have been eliminated in cyclone separator 2i, these can be directed via line 32 by opening valve 33 therein, valve 22 in line 23 being closed, to a crusher and screening device 'shown generally at 34 wherein the coke particles are ground to the proper mesh before returning them to hopper 24 via line 35.

'I'he reaction products from cyclone separator 29 can be directed to any suitable recovery means via line 36 as for example, to an absorption system. A waste heat boiler 31 can suitably be inserted in line 36 to utilize'the heat from the gases prior to absorption; and in addition a water cooler or other cooling means should be used to reduce the temperature of the hydrocarbon stream. The cooled gases are directed to absorber 38, wherein they are contacted with a suitable absorber oil from line 39, the hydrogen with or without methane and other light gases passing overhead through line 40 while the rich absorber oil with its absorbed gases is withdrawn from the base of absorber 38 through line 4I and sent to a stripper or other-means for removing the normally gaseous hydrocarbons therefrom. Alternatively, the gases may be passed through a sulfuric acid absorber and alcohols produced from the olefins in the gases by well-known means. The pressure supplied by pump II in line III should be sufficiently great to force the exit gases in line 3 6 through absorber 38. If necessary. however, a compressor (not shown) can be inserted ln line 36 rto force the gases through the absorption system.

The coke from .cyclone separators 2|, 21 and 29 accumulates in hopper 24 and standpipe 42 below it. The solid particles are preferably maintained in an aerated condition in this standpipe and hopper, gas which can suitably be flue gas, steam or other inert gas being introduced by lines 43 and 44 to hopper 24 and standpipe 42 respectively, the excess gases being vlented through line 45. In this Way Ithe particles are maintained in such condition that they will ow freely and bridging Within hopper 24 and standpipe 42 -is prevented. The aerated coke is fed by its own hydrostatic pressure from the base of standpipe 42 through line 4'5 into a stream of compressed air inline 41 and directed to coke heater 48. A control valve 49 in line 45 regulates the proportion of coke added to the air in line 41. A pump 50 in line 41 supplies fthe necessary compression to the air stream.

Coke heater 48 is a low-velocity upiiow reactor of such dimensions Ias to provide the required residence time for the coke in this zone. In order to operate economlcallyit is desirable to consume the bulk of the oxygen in the compressed Iair stream and therefore it is preferred to allow a long residence time for the coke. This can -be eifected by using a coke heater of such dimensions that the coke settles relative to the upfiowing air stream. Velocities, for example, ofthe order of 0.5 to 2 feet per second can be used. Stated otherwise, coke residence time can be relatively long and of the order of magnitude of 1.5 to 40 times the -air contact time. Coke heater 48 can be a single drum reactor' with or without baiiies or can be separated into rtwo or more large zones, as shown, in order to make the contact stagewise. The oxygen in the air converts at least a part of the car-bon on the coke to the oxides of carbon, the heat generated thereby heating the remainder of the coke to high temperatures of from ll F. to -3000 F. or higher, depending upon the length of time the coke remains in the reactor and the amount of air available. Coke heater 48 i-s preferably well insulated or lined with heat-resistant material, such as fire brick, etc. so that the dissipation of heat -is minimized and the vessel adapted to withstand the elevated temperature encountered.

The hot coke and ilue gas pass overhead from coke heater 48 through insulated line 5| to sepanator 52, the ilue gas passing overhead through line 53 from separator 52, which 'may be a cyclone separator or series of cyclone separators with or without a Cottrell precipitator at the end of the series, where it is discharged from the system. In order to utilize the heat contained in the ue gas stream, it is usually desirable to install a waste heat boiler 54 in line 53 whereby water is converted -to steam for use in general refinery operations.

The coke :from cyclone separator 52 is withdrawn through insulated line 55 and accumulated in storage bin 58 at the top of the standpipe 51 where it is maintained in 'an aerated condition, -aerating fluid being introduced .by lines 58.

Any suitable inert gas, including flue gas. can he used as the aerating uid, and the oif gases from separator 52, either before or after their passage through boiler 54, are particularly useful for this purpose because of their comparatively high temperature. standpipe 51 and storage bin 56 are preferably well lined with heat-resistant m9.-- teri-al in order to avoid heat losses. If desired, a small regulated amount of air or other oxygencontaining gas can be injected with the aerating fluid in line 58 in order to maintain the temperature in standpipe 51. The aerating fluid is vented from storage bin 58 via line 59, which can suitably join line 53A .prior -to waste heat boiler 54. Since the aerating fluid prevents the solid coke particles from bridging, the coke will flow by' its own "hydrostatic pressure through line I5 or line I8, as desired. The coke is preferably at about 1100* to 2500" F. or higher, depending upon the temperature of the feed gases in line I4, the character of the feed, and whether the coke is to be injected into the transfer line or .directly Iinto the cracking chamber. In the event that the same coke particles have become too fine during the restoration, it may be desirable to use a series of cyclone separators and to discard the ne material therefrom.

Depending on the conditions of operation and various other factors, there may be a net loss or a net gain in the amount of coke. 'The coke particles are preferably from about 50 .to 500 mesh, as lthey pass from standpipe 51; i. e., the particles -will pass through a wire screen hav-ing 50 meshes per square inch and be held by a. wire screen having 500 meshes per square inch. Also, a partv of the coke may be crushed -too ne and therefore is discarded from crusher 34. Under these circumstances it becomes necessary to add addition-al solid carbonaceous material, such as coke, coal, etc., which can be injected from line 60 into =line 5|, where i-t commingles with the hot coke and gases from coke heater 48 prior to sep-v aration in cyclone separator 52.

On the other hand, the deposition of carbon during Athe gas cracking may lbuild up the coke to such an extent that there is a net gain over all. Accordingly, a part of the coke should be withdrawn, either continuously or intermittently, which can be done through valved line 5I leading from standpipe 24.

In starting up the unit, the coke in standpipe 51 is necessarily cold or at least below the desired temperature. A by-pass line 62, which leads from line I5 to line 41 is shown, by which the coke can first be directed to coke heater 48 for heating. As soon as the coke in standpipe 51 is at the proper temperature, valve 63 in line 62 is closed and valve I5a in line I5 opened, and the hot feed charge from coils I2 in furnace I3 admitted to line I4. Alternately, storage bin 56 and standpipe 51 can be free of coke, and fresh coke from any suitable source injected from line 64 to line 41 prior to coke heater 48, and' thence by the described means to standpipe 51 where it is used for gas cracking.

From'the above description, it is evident that I have an improved method and means for the conversion of normally. gaseous hydrocarbons to unsaturated hydrocarbons and/or hydrocarbons of lower molecular weight. Coking of tubes and plugging of apparatus is eliminated or at least reduced to a minimum, while the heat requirements are supplied by the injection oi hot solid carbon-aceous material, which is reheated by oxidation with air. By employing low velocity upow reactors, a uniform cracking throughout the reaction chamber is obtained, with a consequent substantial uniformity of products. The heating agent is easily and uniformly restored to temperature in an upfiow low-velocity heater, the apparatus is simple and the control excellent.

Although I have illustrated one embodiment of my invention it should be understood that this is chiey by way of illustration and not by way of limitation, and that obvious equivalents are intended to be included within the scope of my invention. Also, for the sake of simplicity, I have omitted various details, such as automatic control means, heat exchangers, etc., all of which will be readily supplied by one skilled in the art intending to practice my invention.

I claim:

1. The method of cracking normally gaseous hydrocarbons which comprises heating said normally gaseous hydrocarbons to an elevated telnperature below that at which a substantial amount of crackin-g occurs, supplying the heat of cracking to said hydrocarbons by contacting therewith finely divided coke within a reaction zone, maintaining the hydrocarbons and hot coke within said zone in such proportions that the combined stream of hydrocarbons and coke has a temperature within the range of between about 1l00 F. and 1500 F., employing a, hydrocarbon contacting time of between about 0.5 and about 50 seconds within said zone and a. coke residence time within said zone of between about 0.5 and about 100 minutes, the ratio of coke residence time to gas contacting time being in the approximate range of between about 1.5 to 1 and about 40 to 1, continuously separating cracked gases and coke particles, suspending the separated coke in an oxygen-containing gas within a heating zone, consuming a'substantial proportion of the coke by combustion within the heatingzone with the result that the residual coke particles are heated to a high temperature substantially in excess of the cracking temperature, and com-- mingling the hot coke with additional quantities of gaseous hydrocarbons to supply the heat of cracking thereto.

2. The method of cracking propane which comprises heating a. gas stream containing a substantial amount of propane to about 1000 F., injecting coke particles of from to 500 mesh heated to about 1800 F. into said hot gas stream in such proportion .that the commingled streams have a temperature of about 15009 F., maintaining said cokeparticles dispersed in said gas stream in a commingled state within a low velocity upflow reaction zone until a gas contact time of about 5 seconds has been obtained and substantial .cracking of the propane has occurred, continuously separating the cracked propane and said coke, heating said coke particles by dispersing in a stream of air to eect partial combustion of the said coke in a separate zone and thereby attaining a temperature of at least 1800 F., and returning the residual hot coke particles to the said propane cracking step.

MAURICE H. ARVESON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

